1. Field of the Invention
The present invention relates generally to the field of fiber optics, and in particular to systems and techniques for terminating an optical fiber with low backreflection.
2. Background Art
In a fiber-optic sensor, a reference light is launched into a sensor fiber and the resulting backreflected light is detected and analyzed. A fiber-optic sensor can be used, for example, to detect changes in shape and/or position based on the measurement of the amplitude/phase of distributed backreflection along the length of the sensor fiber. In a shape/position sensor, and in other sensor applications, it is of utmost importance to suppress backreflection coming from the tip of the sensor fiber. A strong reflection from the tip can adversely saturate the detector used to detect distributed backreflection, and consequently impair the accuracy and resolution of the shape/position sensing.
While conventional methods for terminating a sensor fiber are capable of reducing backreflection from the fiber tip, the sensor performance can become highly sensitive to touch or to backreflection or backscattering from nearby objects. In a shape-sensing application, a sensor fiber is typically deployed within a tightly confined region. Light emitted from the tip of the sensor fiber can be reflected by the surfaces of the confined region back into the sensor fiber and coupled into the fiber core. Furthermore, the properties of the medium surrounding the tip can alter the behavior of light at the tip, increasing reflectivity, for example.
One way to overcome this problem is to attach a backreflection-suppression element, such as a light-absorbing glass rod or fiber, to the tip of the sensor fiber. Light emitted from the sensor fiber tip propagates through the absorbing structure, and is attenuated by such an extent that reflection from nearby objects becomes insignificant. However, the addition of the light-absorbing structure poses a number of problems.
First, the attachment of a light-absorbing structure to the fiber tip affects the positional accuracy of the sensor because of the lack of a sensing element (e.g., Bragg grating) in the appended structure. The light-absorbing structure has to have a length of at least a few millimeters in order to provide sufficient suppression of light emitted from the fiber tip. Thus, the positional accuracy of the sensor tip is limited by the length of the added structure.
Second, the light-absorbing structure typically has a refractive index that is different from that of silica glass fiber. Therefore, in order to suppress backreflection from the interface between the sensor fiber and the light-absorbing structure, a special splicing technique is needed, involving the use of a thermal diffusion technique to achieve index matching. The special splicing technique calls for individual processing of each fiber sensor, thus making batch processing difficult. Further, the strength of the sensor tip is compromised. The presence of the spliced component means that the sensor tip is increasingly prone to breakage/fracture as it is passed through tight bends.